The present invention relates to a method and device for the measurement of arterial blood pressure (ABP). More particularly, the present invention relates to a method and a device for measuring systolic and diastolic blood pressure.
Blood supply to the tissues of a living body is essential for maintaining their metabolism and proper function. During systole (heart contraction), blood is ejected from the heart into the arterial system, thereby increasing the arterial blood pressure. The maximal arterial blood pressure (ABP) is the systolic blood pressure (SBP). During and after systole, blood flows from the arteries, through the capillaries, into the veins, and from them back into the heart. The period between two systoles is called diastole. During diastole, the arterial blood pressure decreases; the minimal arterial blood pressure (at the end of diastole) is called diastolic blood pressure (DBP).
Similar to blood pressure, blood volume in the tissue also shows oscillations at the heart rate. During systole, blood is ejected from the left ventricle into the peripheral tissues, thereby increasing their blood content. The measurement of the cardiac induced changes of tissue blood volume is called plethysmography, which can be performed by means of several methods, including photoplethysmography (PPG), which is the measurement of light absorption in tissue. The PPG signal originates from the increase of tissue blood volume during systole, and the consequent higher light absorption. FIG. 1 shows a known PPG probe attached to a finger. The light source L emits light into the tissue and the photodetector D measures the light scattered from the tissue under the skin. The output of the photodetector depends on the tissue blood volume, and oscillates with the oscillations of the latter.
FIG. 2 shows the blood pressure and the PPG signal measured simultaneously in the finger arteries as a function of time. The blood pressure measurement was performed on a fingertip by means of a continuous, non-invasive blood pressure meter (Finapres, Ohmeda, U.S.A.). As can be seen in FIG. 2, the curve of oscillations (at the heart rate) of the tissue blood volume as measured by the PPG signal, is similar, but not identical, to the ABP curve.
Blood pressure can change because of exercise, mental stress, or excitement. It also changes spontaneously due to activity of the autonomic nervous system. For adults aged below 40 years, the values of normal blood pressure (at rest) are 120 mmHg and 80 mmHg for systolic and diastolic blood pressures, respectively; If the ABP is too high (hypertension), the subject is at higher risk of cerebral stroke and heart attack. Lower than normal blood pressure (hypotension) is acutely hazardous, since it may cause low blood supply to the brain, resulting in fainting or even in brain damage. Decreasing blood pressure for patients after trauma, surgery or heart attack is an indication of cardiovascular deterioration.
Blood pressure can be measured invasively by inserting a catheter into an artery and measuring the pressure by means of a piezoelectric device. This measurement is the most reliable one, and it is done in intensive care units where an arterial line is inserted for additional purposes. Due to its invasiveness, this method is not used for routine applications.
The auscultatory method is the most common method for non-invasive measurement of blood pressure, and is based on hearing (via stethoscope or microphone) the turbulence sounds which appear in a compressed artery when it is intermittently closed and opened by means of an inflatable cuff having air pressure of a value between that of diastolic and systolic blood pressure. Usually, the cuff air pressure is increased above the SBP, then decreased. The cuff air pressure at which the turbulence sounds appear is the SBP; the pressure at which the quality of the sounds changes, becoming muffled, is defined as the higher DBP (IV Korotkoff or phase IV DBP); the pressure at which they totally disappear is defined as the lower DBP (V Korotkoff or phase V DBP). In general the lower DBP has to be taken as the DBP, but for some groups of patients for whom the Korotkoff sounds are heard even for extremely low cuff pressure, such as in pregnant women, the higher DBP is taken. The manual auscultatory method (using a stethoscope) has been accepted as the gold standard for non-invasive ABP measurement, and is routinely used in clinics and hospitals. The automatic auscultatory method (using a microphone), is also used for monitoring ABP in hospital wards. Despite its extensive use, the auscultatory method is not accurate, both because of the difficulty in detecting the correct sounds and because of the unclear relationship between the disappearance of the turbulence sounds and DBP.
Automatic blood pressure measurement can also be done by means of the oscillometric method. A cuff is applied to the arm or finger and, besides the measurement of the average air pressure, the oscillatory variations of air pressure in the cuff are measured by means of a piezoelectric pressure transducer. Oscillations at the rate of the heart can then be seen in the cuff pressure (oscillometry) due to the cardiac induced changes in the arterial blood volume. In an alternative method, a sensor for detecting blood volume changes in the arteries, such as a PPG device, is attached to the skin under the cuff. Here too, oscillations at the rate of the heart appear in the volume sensor output (volume oscillometry). When the air pressure is continuously increased above diastolic blood pressure, these oscillations also increase until the air pressure is equal to the mean blood pressure, and then they decrease. The systolic and diastolic blood pressure can be derived from the curve of the amplitude of oscillation as a function of the air pressure, using empirical formulae. This method, which is called "oscillometry", can be used for monitoring blood pressure, but the measurement time is long: more than 20 heart beats, depending on the patient and on the required accuracy. In any case, the method and the commercial devices which are based thereon are not considered to be accurate.
The low accuracy of the automatic auscultatory and oscillometry methods for the measurement of diastolic and systolic blood pressure, and the need for a reliable automatic method, have resulted in several attempts to develop other methods for blood pressure measurements. Some of these methods are based on PPG measurement. The systolic blood pressure can be non-invasively measured by means of PPG, by using a PPG device and a cuff around the arm or finger, increasing the air pressure in the cuff, and determining the air pressure at which the PPG signal disappears. This air pressure is equal to the systolic blood pressure in the artery under the cuff. In principle, measurement of systolic blood pressure by PPG and a pressure cuff may be performed in a straightforward manner by identifying the onset of PPG pulses. Determination of the diastolic blood pressure from the PPG signal, on the other hand, is more difficult.
In U.S. Pat. No. 5,269,310, there is disclosed a method for measuring, by means of PPG, changes of blood volume in the arteries during systole together with the patient's blood pressure, and for determining what is assumed to be a constant k particular to the patient's arterial blood pressure-volume relationship. By means of this calibration, the DBP and SBP for each heartbeat is determined from the minimum and maximum points of the PPG signal. The method is not accurate, since DBP and SBP are not actually related to the maximum and minimum of the PPG signal by a constant k.
In U.S. Pat. No. 5,423,322, an exponential relationship is assumed between the ABP and the blood volume changes measured by PPG, for the assessment of the cardiac-induced blood pressure oscillations from the simultaneous blood volume oscillations in the heart rate. There are several drawbacks to this method, as will now be detailed.
Firstly, the relationship between the arterial blood pressure and the blood volume is not strictly exponential. In fact, the volume vs. pressure curve changes as a function of time, and even changes between the period of increasing pressure (systole) to the period of decreasing pressure (diastole) within the same cardiac cycle, as can be seen in FIG. 2 of the present application.
Secondly, the blood volume changes not only in a single artery, but also in the small arteries and in the arterioles (resistance vessels). It is not possible to simulate the entire group of arteries and arterioles as a single artery, since the pressure therein is not constant due to the reduction of the blood pressure from the arteries to the arterioles.
Another known method for continuous measurement of finger ABP is the arterial volume clamp method, which is based on PPG. The device utilized for this method is composed of a finger cuff with a PPG probe, and the method is based on the determination of the cuff air pressure which is required to keep the arterial blood volume constant. The device enables the measurement of ABP changes during the cardiac cycle via very rapid changes of the cuff air pressure. The method is very sophisticated, but it was not found to reliably record ABP. The device is expensive, due to the need to swiftly change the cuff air pressure in accordance with the blood pressure changes during the cardiac cycle.
Other methods for the measurement of ABP have been suggested, but the only methods which have been accepted for routine and comprehensive clinical use are the oscillometric and auscultatory methods, indicating that the other suggested methods are either not reliable enough, or are too complicated, for clinical use.
Another approach, suggested by a number of academic papers but not implemented in practice, proposes to measure diastolic blood pressure on the basis of a delay in the pulse caused by pressure from a cuff. Applying a pressure between diastolic and systolic blood pressure on an artery results in compression of the artery for part of the cardiac cycle time as can be seen in FIG. 3. As a result, the pressure pulse in the artery distal to the pressure application location will start later than in contralateral arteries not affected by the pressure. This approach was presented by L. A. Geddes et al. in a paper entitled "Pulse Arrival Time as a Method of Obtaining Systolic and Diastolic Blood Pressure Indirectly", (Medical & Biological Engineering & Computing, September 1981, 19:pp. 671-672). Geddes et al., experimenting on dogs, compared the measurements of an invasive pressure sensor in the leg of a dog with either another similar sensor in the contralateral leg or an ECG reference to detect a delay in the pulse reaching a location beyond a pressure cuff. The use of ECG as a time reference is particularly problematic, giving broad scattering of results. The measurement of the pulse delay due to the cuff pressure using ECG as a reference was also suggested by A. Marmor et al. (Clin. Cardiol. 1987, 10:215-221) and T. Sharir et al. (Hypertension 1993, 21: 74-82) for the determination of the systolic increase curve of arterial pressure as a function of time.
Even with a second sensor in the contralateral leg as a reference, the point at which the diastolic pressure is supposedly indicated appears poorly defined. Furthermore, the measurement of the time delay between the pulses in the two sides is inaccurate, since the time delay was identified as the time difference between the minima of the corresponding pressure pulses in the two sides. The measurement of the time of the pulse minimum is subject to significant error, since the curve in the neighborhood of the minimum changes slowly and a small error in the pressure measurement may result in a large error in the determination of the minimum time. This problem would be accentuated if the noninvasive method were used for the determination of the start of the pulse, in a peripheral region, since noninvasive measurement of any parameter which is related to the pulse pressure has a higher noise level than direct invasive measurement of arterial blood pressure. It should be noted that Geddes et al. claimed that it is possible to use a noninvasive technique to detect the start of the pressure pulse, and that they intend to do that, and to publish the results in a second paper. However, to the best of our knowledge, no such paper has ever been published. It seems that the analysis of the non-invasively achieved signal as suggested by Geddes did not permit accurate measurement of the diastolic blood pressure, possibly for the reasons discussed above.
There is therefore a need for a practical, non-invasive technique for measuring arterial diastolic blood pressure on the basis of a delay in pulses caused by a pressure cuff. It would also be highly advantageous to provide an automatic device for measuring arterial diastolic blood pressure according to such a technique.